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J. Am. Chem. Soc. 1998, 120, 9838-9843
A Photoresponsive Silicon Radical within a Porphyrin π-Cloud: Photolysis of Organo- and Nitroxysilicon Porphyrins with Visible Light Jian-Yu Zheng, Katsuaki Konishi, and Takuzo Aida* Contribution from the Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 Japan ReceiVed January 26, 1998 Abstract: Upon irradiation with visible light (λ > 420 nm) in C6D6 under Ar, dialkylsilicon porphyrins Si(TPP)R2 (TPP ) 5,10,15,20-tetraphenylporphinato) (R ) CH2CH2CH3 (1) and CH2TMS (2)) reacted with nitroxy compounds, such as 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidinyl-1-oxy, and 1,1,3,3-tetramethylisoindolinyl-2-oxy, to give dinitroxysilicon porphyrins 6. In the absence of nitroxy compounds, photolysis of 1 resulted in the formation of a long-lived (>50 days at 25 °C) EPRactive silicon diradical with a g-value of 2.0026, which did not react with nitroxy compounds in the dark but was smoothly trapped by TEMPO to give 6a upon excitation with visible light. Irradiation of 6 with visible light in the presence of free nitroxy compounds resulted in reversible axial ligand exchange, which proceeded, in response to the switching on and off of the light, to reach an equilibrium.
Introduction Photochemical properties of porphyrin complexes have attracted continuing attention not only in relation to biological photosynthetic reactions but also for molecular design of lightharvesting materials, photoresponsive switches, photocatalysts, and so forth. Although extensive studies have been made on photoexcited metalloporphyrins with a variety of transition and nontransition metals,1,2 the photochemical behaviors of porphyrin complexes of nonmetal elements are much less explored to date.3,4 Silicon porphyrin is a representative of nonmetal porphyrin complexes,3,5 which accommodates a hexacoordinate silicon atom at a geometric center of four pyrrole nitrogen atoms. Very recently, we have succeeded in the synthesis of a series of organosilicon porphyrins (1-5) and have found by crystallographic studies that their two axial ligands are in a trans configuration to each other.6 Although the porphyrin rings in silicon porphyrins are highly nonplanar when the axial ligands * Corresponding author: (phone, fax) +81-3-5802-3363; (e-mail)
[email protected]. (1) For selected reviews, see: (a) Porphyrins: Excited States and Dynamics; Gouterman, M., Rentzepis, P. M., Straub, K. M., Eds.; American Chemical Society; Washington, DC, 1986. (b) Guilard, R.; Kadish, K. M. Chem. ReV. 1988, 88, 1121. (c) Suslick, K. S.; Watson, R. A. New J. Chem. 1992, 16, 633. (d) Maldotti, A.; Amadelli, R.; Bartocci, C.; Carassiti, V.; Polo, E.; Varani, G. Coord. Chem. ReV. 1993, 125, 143. (e) Vicek, A. Chemtracts: Org. Chem. 1996, 9, 322. (2) For photodissociation of axial ligands of metalloporphyrins, see: (a) Kadish, K. M.; Xu, Q. Y.; Barbe, J.-M.; Guilard, R. J. Am. Chem. Soc. 1987, 109, 7705. (b) Hirai, Y.; Murayama, H.; Aida, T.; Inoue, S. J. Am. Chem. Soc. 1988, 110, 7387. (c) Miaya, G. B.; Barbe, J.-M.; Kadish, K. M. Inorg. Chem. 1989, 28, 2524. (d) Balch, A. L.; Cornman, C. R.; Olmstead, M. M. J. Am. Chem. Soc. 1990, 112, 2963. (3) Kadish, K. M.; Xu, Q. Y.; Barbe, J.-M.; Guilard, R. Inorg. Chem. 1988, 27, 1191. (4) Segawa, H.; Nakamoto, A.; Shimizu, T. J. Chem. Soc., Chem. Commun. 1992, 1066. (5) (a) Kane, K. M.; Lemke, F. R.; Petersen, J. L. Inorg. Chem. 1995, 34, 4085. (b) Kane, K. M.; Lemke, F. R.; Petersen, J. L. Inorg. Chem. 1997, 36, 1354. (6) Zheng, J.-Y.; Konishi, K.; Aida, T. Inorg. Chem. 1998, 37, 2591.
are of electron-withdrawing character such as fluoride and triflate,5 these organosilicon porphyrins have a strong tendency to adopt a planar conformation as for the porphyrin ring, indicating an electronic interaction between the axial ligands and the porphyrin ring via the central silicon atom. In the course of this study, we noticed that some of the organosilicon porphyrins are labile under illumination with room light. This observation prompted us to investigate the photochemical properties of organosilicon porphyrins. In the present paper, we report results of studies on the photochemical reaction of a series of organosilicon porphyrins 1-5 with radical scavengers such as nitroxy compounds and highlight the formation of a long-lived silicon diradical with a unique photoswitchable reactivity. Results and Discussion (I) Reaction of Organosilicon Porphyrins with Nitroxy Compounds upon Irradiation with Visible Light. Irradiation of dialkylsilicon porphyrins (Si(TPP)R2, 1 and 2; TPP ) 5, 10,15,20-tetraphenylporphinato) with visible light in the presence of nitroxy compounds resulted in a quantitative formation
S0002-7863(98)00273-X CCC: $15.00 © 1998 American Chemical Society Published on Web 09/11/1998
PhotoresponsiVe Si Radical in a Porphyrin π-Cloud Scheme 1
of the corresponding dinitroxide complexes 6 (Scheme 1).7,2 For example, when a mixture of dipropyl complex (16 (5.0 µmol) and 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) (20 µmol)
in C6D6 (0.5 mL) under Ar was exposed to a xenon arc light (λ > 420 nm) at 25 °C for 20 min, 1H NMR signals due to the axial propyl groups of 1 [δ -1.38 (f, CH3), δ -4.24 (e, MeCH2), δ -6.51 (d, CH2Si(TPP)); Figure 1a] disappeared completely. On the other hand, new signals assignable to a bis(TEMPO) complex 6a [δ -1.24, -2.41 (g, (CH3)2C-NOSi(TPP))] appeared together with a set of new signals due to [1-(1′-propoxy)2,2,6,6-tetramethylpiperidine [Pr-TEMPO; δ 4.03 (D, EtCH2ON)] (Figure 1c), which was identified by comparison with the authentic sample prepared from TEMPO and 1-propylmagnesium bromide.8 From the relative intensity of the signal at δ 4.03 (Pr-TEMPO) to that at δ -1.24 (6a), Pr-TEMPO was found to be formed twice as much as 6a, as illustrated in Scheme 1. The trans configuration of the two TEMPO ligands in 6a with respect to the silicon atom is evident from a single signal due to the ortho protons (δ 8.32) of the meso phenyl groups of the porphyrin ring, as in cis complexes of tetravalent metalloporphyrins, the ortho protons on the coordinated and noncoordinated faces of the porphyrin ring are not equivalent to each other and provide two signals.9 In sharp contrast, the reaction of 1 with TEMPO in the dark hardly took place even at 90 °C for 168 h, indicating that photoexcitation of the porphyrin ring of 1 is essential for the formation of 6a. When the reaction upon selective excitation of 1 at its Q-band (638 ( 10 nm) was followed by means of UV-visible spectroscopy ([1]0/[TEMPO]0 ) 0.1/0.4 mM), characteristic absorption bands at 455.0 and 656.5 nm decreased in intensity with time, while two new absorption bands assignable to 6a appeared at 425.5 and 561.0 nm and became predominant (Figure 2).10 From the change in absorbance at 656.5 nm, the conversion of 1 during the initial (7) For cleavage of Si-C bonds in silicon compounds, see: (a) Reference 3. (b) Tamao, K.; Akita, M.; Kato, H.; Kumada, M. J. Organomet. Chem. 1988, 341, 165 and references therein. (8) (a) Bolsman, T. A. B. M.; Blok. A. P.; Frijins, J. H. G. Recl. TraV. Chim. Pays-Bas 1978, 97, 313. (b) Martin, B. R.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 585. (9) (a) Shibata, K.; Aida, T.; Inoue, S. Chem. Lett. 1992, 1173. (b) Sawson, D. Y.; Sangalang, J. C.; Arnold, J. J. Am. Chem. Soc. 1996, 118, 6082.
J. Am. Chem. Soc., Vol. 120, No. 38, 1998 9839 30-s irradiation was determined to be 23%, from which the quantum yield of the reaction was evaluated to be 13%. The UV-visible spectral profile in Figure 2 also showed the appearance of a shoulder around 450 nm, assignable to the Soret band of monopropyl mono-TEMPO complex 6a′, which however decayed upon further irradiation. In the 1H NMR spectrum of the reaction mixture ([1]0/[TEMPO]0 ) 10/40 mM) after 4-min irradiation (Figure 1b), signals assignable to 6a′ were observed at δ -1.33, -2.47 (g′, (CH3)2C-NOSi(TPP)), δ -1.15 (f ′, CH3CH2), -4.00 (e′, MeCH2), and -6.09 (d′, CH2Si(TPP)). Plots of the mole fractions of 6a and 6a′, as determined from the intensities of the corresponding signals (δ -4.00 and -1.24), versus irradiation time (Figure 3) show that the yield of 6a′ once reached a maximum in 4-min irradiation and then decreased, while that of 6a increased sigmoidally with irradiation time. Therefore, the formation of dinitroxide complex 6a from 1 and TEMPO is likely to occur via a stepwise mechanism involving 6a′ as intermediate. In addition to TEMPO, other nitroxy compounds such as 4-oxo-2,2,6,6-tetramethylpiperidinyl-1-oxy (4-oxo-TEMPO) and 1,1,3,3-tetramethylisoindolinyl-2-oxy (TEMIO) also reacted with dipropylsilicon porphyrin (1) ([1]0/[4-oxo-TEMPO or TEMIO]0 ) 10/40 mM), upon irradiation with visible light (λ > 420 nm) for 30 min, to give quantitatively the corresponding silicon dinitroxide complexes 6b [δ -1.59, -2.08 ((CH3)2C-NOSi(TPP)), 8.24 (o-H)], and 6c [δ -1.11, -1.63 ((CH3)2C-NOSi(TPP)), 8.40 (o-H)]. As for other organosilicon porphyrins, a dialkyl complex such as 2 (R ) CH2SiMe3) also reacted with TEMPO ([2]0/[TEMPO]0 ) 10/40 mM) under irradiation with visible light (λ > 420 nm), giving 6a in a quantitative yield in 30 min. On the other hand, no reaction took place with organosilicon porphyrins bearing axial Si-C (sp2) and Si-C (sp) bonds such as 3 (R ) CHdCH2),6 4 (C6H5),6 and 5 (Ct CC6H5)6 under similar conditions. In relation to the low reactivities of 3 - 5 toward TEMPO, our previous study has shown that the Si-C bond distances in 3 and 5 (1.83 and 1.82 Å) are shorter than that in 2 (1.95 Å), suggesting a possible dπ-pπ interaction between the central silicon atom and the unsaturated axial ligands.6,11 (II) Photolysis of Dipropylsilicon Porphyrin (1) in the Absence of Nitroxy Compounds. As described in the above section, the photochemical generation of silicon dinitroxides 6a6c from dialkylsilicon porphyrins 1 and 2 and nitroxy compounds is accompanied by the simultaneous formation of 2 equiv of O-alkylated nitroxy compounds (4, Figure 3). This observation indicates that the axial Si-C bond is cleaved homolytically to generate a (porphinato)silyl radical and an alkyl radical, both of which are subsequently trapped by the nitroxy compound. In relation to this mechanism, we investigated photolysis of Si(TPP)Pr2 (1) in the absence of nitroxy compounds: When a C6D6 solution of 1 (10 mM) was irradiated with visible light (λ > 420 nm) at 25 °C under Ar, the color of the solution gradually turned from green to brown. TLC trace of the reaction mixture on a silica gel plate with CH2Cl2 as eluent showed a complete disappearance of 1 (Rf ) 0.7) within 10 min to give a major spot with an Rf of 0.05. The 1H NMR spectrum of the reaction mixture showed a complete disappearance of the signals at an upfield region (δ -1 to -7 ppm) due to the axial propyl groups, indicating the photolysis of the axial Si-C bonds of 1. On the other hand, absorption spectral trace of the reaction in C6H6 (10) The spectral profile is very similar to those of dialkoxysilicon porphyrins. (11) For dπ - pπ interactions in silicon compounds, see: Buell. G. R.; Corriu, R.; Guerin, C.; Spialter, L. J. Am. Chem. Soc. 1970, 92, 7424.
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Figure 1. Reaction of Si(TPP)Pr2 (1, 10 mM) with TEMPO (40 mM) in C6D6 at 25 °C under Ar. 1H NMR spectra before irradiation (a) and after irradiation with visible light (λ > 420 nm) for 4 (b) and 20 min (c). Free TEMPO is NMR-silent.
([1]0 ) 0.1 mM) at 25 °C upon selective excitation at 638 ( 10 nm showed that the Q-band at 656.0 nm, characteristic of 1, disappeared in 1 min, while the Soret band at 455.0 nm shifted to 425.5 nm. Furthermore, an absorption band at 444.5 nm, assignable to an intermediate monopropyl complex, was observed at the initial stage of the reaction (∼10 s), suggesting that the photolysis of 1 in the absence of nitroxy compounds also occurs in a stepwise fashion. The EPR spectrum of a C6D6 solution of 1 (10 mM) after 10-min irradiation displayed a strong single band with a g-value of 2.0026 at 25 °C (Figure 4a). Although this signal was significantly broad without any hyperfine splitting, the g-value is characteristic of those reported for silyl radicals (2.0022.006).12 Quite interestingly, the EPR-active species lasted for a long period of time without significant decay at 25 °C in the dark, so long as the reaction mixture was kept under Ar: The signal retained 95 and 86% of the original intensity in 1 and 50 (12) Chatgillialoglu, C. Chem. ReV. 1995, 95, 1229.
days, respectively (Figure 4b,c). Of further interest to note is the fact that this EPR-active species does not react with TEMPO in the dark: Upon addition of 10 equiv of TEMPO with respect to 1 after the complete photolysis of 1 in C6D6, no 1H NMR signals due to 6a were detected throughout observation for 4 h in the dark. However, upon exposure of this mixture to visible light (λ > 420 nm) for 10 min, 6a was formed in 43% yield.13 The formation of bis(TEMPO) complex 6a, thus observed, indicates that the EPR-active species, generated upon photolysis of 1, is most likely to be a silicon diradical. However, considering its low reactivity toward TEMPO in the dark, the unpaired electrons at the ground state are considered to be partially delocalized over the surrounding π-cloud of the porphyrin ring (7, Scheme 2). On the other hand, when the porphyrin ring is excited with visible light, such an electronic delocalization may be switched off, resulting in the generation (13) Determined from the 1H NMR signal intensity at δ -1.24 (12H) relative to that of the pyrrole-β protons (8H).
PhotoresponsiVe Si Radical in a Porphyrin π-Cloud
Figure 2. Reaction of Si(TPP)Pr2 (1, 0.1 mM) with TEMPO (0.4 mM) in C6H6 at 25 °C under Ar. Absorption spectral change upon irradiation with 150-W xenon arc light at 638 ( 10 nm.
J. Am. Chem. Soc., Vol. 120, No. 38, 1998 9841
Figure 4. EPR spectra at 25 °C of a C6D6 solution of 1 (10 mM) kept in the dark for 10 min (a), 24 h (b), and 50 days (c) after irradiation with visible light (λ > 420 nm) for 10 min at 25 °C under Ar.
Scheme 2
Scheme 3
Figure 3. Reaction of Si(TPP)Pr2 (1, 10 mM) with TEMPO (40 mM) in C6D6 at 25 °C upon irradiation with visible light (λ > 420 nm) under Ar. Time courses of the formation of monopropyl mono-TEMPO complex (6a′, 9), bis(TEMPO) complex (6a, b), and Pr-TEMPO (4), as evaluated from the intensities of the 1H NMR signals at δ -2.41 (12H) for 6a, δ -4.00 (2H) for 6a′, and δ 4.03 (2H) for Pr-TEMPO, relative to the sum of those of the pyrrole-β protons (δ 9.1-9.3, 8H).
of a silicon-centered diradical species with sufficient reactivity toward TEMPO. Therefore, the silicon diradical within a porphyrin π-cloud is a “photoswitchable radical”, whose reactivity can be controlled by visible light. (III) Reaction of Dinitroxysilicon Porphyrins with Nitroxy Compounds. To obtain further insight into the photochemical properties of the (porphinato)silyl radical, we investigated photolysis of dinitroxysilicon porphyrin 6a in the presence of free nitroxy compounds. Quite interestingly, the axial nitroxy
groups in 6a underwent ligand exchange with the nitroxy compounds, for which the effect of visible light was again essential (Scheme 3): When a C6D6 solution of 6a containing a mixture of TEMPO and 4-oxo-TEMPO ([6a]0/[TEMPO]0/[4oxo-TEMPO]0 ) 10/40/60 mM) was irradiated with visible light (λ > 420 nm) at 25 °C under Ar, the 1H NMR signals due to the methyl groups in the nitroxy ligands of 6a decreased with time, while new signals at δ -1.59 and -2.08 assignable to those of a 4-oxo-TEMPO group attached to Si(TPP) (4-oxoTEMPO-Si(TPP)) appeared. As shown in Figure 5 (0), the mole fraction of 4-oxo-TEMPO-Si(TPP) reached a plateau at 30% within 30-min irradiation. When 6b was used in place of 6a for the above reaction ([6b]0/[TEMPO]0/[4-oxo-TEMPO]0 ) 10/60/40 mM), a similar photoinduced ligand exchange occurred, where the mole fraction of the TEMPO ligand reached a plateau at 70% within 30 min. Therefore, the photoinduced exchange of the nitroxy groups is fully reversible. In sharp contrast, no exchange of the axial nitroxy ligands occurred in the dark throughout an observation over a period of 1 h under otherwise conditions identical to the above (9). The essential effect of visible light was nicely demonstrated by a light on
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Zheng et al. controlled by visible light. Studies on reactivities of dialkyland dinitroxysilicon porphyrins with unsaturated compounds may be one of the subjects worthy of further investigation. Experimental Section
Figure 5. Irradiation of Si(TPP)(TEMPO)2 (6a, 10 mM) with visible light in the presence of a mixture of TEMPO and 4-oxo-TEMPO ([6a]0/ [TEMPO]0/[4-oxo-TEMPO]0 ) 10/40/60 mM) in C6D6 at 25 °C under Ar. Changes in mole fraction of the 4-oxo-TEMPO group attached to Si(TPP) ([4-oxo-TEMP-Si(TPP)]/([TEMPO-Si(TPP)] + [4-oxoTEMPO-Si(TPP)])) under irradiation (0), in the dark (9), and under light on/off conditions (O, b).
(O)/off (b) experiment (Figure 5), where the ligand exchange proceeded only when the reaction mixture was exposed to visible light, whereas the exchange totally stopped when the light was switched off. The photoinduced ligand exchange of the axial nitroxy groups, thus observed, is considered to occur via homolysis of the Si-O bonds in the nitroxide complex, followed by competitive recombination of the resulting (porphinato)silyl radical with free nitroxy compounds (TEMPO and 4-oxo-TEMPO). Therefore, the nitroxide complexes 6 are silyl radical equivalents, which can reversibly photodissociate into free radicals upon excitation of the porphyrin ring. The cleanness of the reaction in Figure 5 suggests a long lifetime of the silyl radical intermediate. Conclusion Silylenes including silicon diradicals with unpaired electrons14,15 have attracted considerable attention not only from a fundamental interest in relation to carbenes16 but also as new potential intermediates in organic synthesis and materials science. In the present study, we have demonstrated that dialkylsilicon porphyrins, upon excitation of the porphyrin ring with visible light, undergo homolytic cleavage of the axial Si-C bonds to generate a highly stable silicon diradical at the ground state, which can be activated by visible light to react with nitroxy compounds. Furthermore, we have also shown that the axial Si-O bonds in the resulting dinitroxysilicon porphyrins are cleaved homolytically to afford free radicals upon irradiation with visible light. These results indicate an interesting possibility that the silicon diradical within a porphyrin π-cloud serves as a “photoswitchable radical”, whose reactivity can be (14) Only ground-state singlet silylenes are known: (a) Gaspar, P. P.; Holten, D.; Konieczny, S. Acc. Chem. Res. 1987, 20, 329. (b) Ando, W. Pure Appl. Chem. 1995, 67, 805. (c) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (d) Tokitoh, N.; Suzuki, H.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 10428. (15) Ground-state triplet silylenes have only been predicted theoretically by computational studies: (a) Gordon, M. S. Chem. Phys. Lett. 1985, 144, 138. (b) Grev. R. G.; Schaefer, H. F.; Gaspar, P. G. J. Am. Chem. Soc. 1991, 113, 5638. (c) Cramer, C. J.; Falvey, D. E. Tetrahedron Lett. 1997, 38, 1515. (16) Carter, E. A.; Goddard, W. A. J. Chem. Phys. 1988, 88, 1752 and references therein.
General Information. Organosilicon porphyrins 1-5 were synthesized according to the method reported in a previous paper.6 Nitroxy compounds such as TEMPO and 4-oxoTEMPO were purchased from Tokyo Kasei and used as received. TEMIO was synthesized according to the literature method17 [mp 130-131 °C (lit.17 mp 128-129 °C). Anal. Calcd for C12H16NO: C, 75.75; H, 8.48; N, 7.36. Found: C, 75.19; H, 8.38; N, 7.53. Pr-TEMPO was synthesized from TEMPO and 1-propylmagnesium bromide according to the literature method.8 1H NMR (C6D6): δ 4.03 (t, 2H, CH2-O), 1.8-1.3 (m, overlapped, 20H, C(CH2)3C, C(CH3)2, and CH2CH3), 1.16 (t, 3H, CH2CH3). C6H6 and C6D6 were distilled over sodium wire in the presence of benzophenone ketyl and stored under Ar. 1H NMR spectra were recorded in C6D6 at 25 °C on a Jeol type GSX-270 spectrometer operating at 270 MHz, where chemical shifts (ppm) were determined with respect to residual C6D5H (δ 7.40) as internal standard. UV-visible spectra were recorded in C6H6 at 25 °C on a Jasco type V-560 spectrophotometer. EPR spectra were recorded at 25 °C on a Jeol FE-1X X-band spectrometer using following parameters: magnetic field, 3280 ( 50 G; modulation, 100 kHz and 0.063 G; amplitude, 5; response, 0.3 s. Photolysis of Organosilicon Porphyrins in the Presence of Nitroxy Compounds. Typically, to a 10-mL round-bottomed flask, fitted with a three-way stopcock, containing dipropylsilicon complex 1 (14.5 mg, 20 µmol) was added a C6D6 solution (2 mL) of TEMPO (12.5 mg, 80 µmol), and the mixture was stirred in the dark at room temperature for 5-10 min. An aliquot (0.8 mL) of the above solution was transferred to a NMR tube, which was then sealed off. The NMR tube was exposed to a 150-W xenon arc light from a distance of 8 cm at 25 °C through a thermal-cut filter and a glass filter to cut out light with a wavelength shorter than 420 nm. The reaction was followed by means of 1H NMR spectroscopy. 6a. 1H NMR: δ 9.11 (s, 8H, pyrrole-β), 8.32 (br, 8H, Pho), 7.7-7.5 (br, 12H, Ph-m, p), 0.75-0.15 (br, 12H, C(CH2)3C), -1.24 and -2.41 (s × 2, 24H, (CH3)2C-NOSi(TPP)). 6b. 1H NMR: δ 9.07 (s, 8H, pyrrole-β), 8.24 (br, 8H, Pho), 7.7-7.5 (br, 12H, Ph-m, p), 0.75-0.15 (br, 8H, C(CH2)CO), -1.59 and -2.08 (s × 2, 24H, (CH3)2C-NOSi(TPP)). 6c. 1H NMR: δ 9.20 (s, 8H, pyrrole-β), 8.40 (d, 8H, Ph-o), 7.7-7.5 (br, 12H, Ph-m, p), 6.70 and 6.22 (br × 2, 8H, C6H4), -1.11 and -1.63 (s × 2, 24H, (CH3)2C-NOSi(TPP)). Determination of Quantum Yield. To a quartz cell of 1-cm path length, fitted with a three-way stopcock, were successively added a mixture of 1 (10 mM) and TEMPO (40 mM) in C6D6 (0.02 mL) and dry C6H6 (2 mL) under Ar. Then, the cell was exposed to a 150-W xenon arc light from a distance of 8 cm at 25 °C through a band-path filter (λ ) 638 nm, bandwidth 10 nm) coupled with UV-cut ( 420 nm) in a manner similar to the above, to give a solution of 6a (40 µmol) containing free TEMPO (160 µmol). To this solution was added a C6D6 solution (0.2 mL) of 4-oxo-TEMPO (40.8 mg, 240 µmol), and an aliquot (0.5 mL) of the mixture was transferred to a NMR tube, which was then sealed off. The tube was exposed to a xenon arc light (150 W) at 25 °C, and the reaction was followed by means of 1H NMR spectroscopy. The mole fraction of the 4-oxo-TEMPO group attached to Si(TPP) ([4-oxo-TEMP-Si(TPP)]/([TEMPO-Si(TPP)] + [4-oxoTEMPO-Si(TPP)])) was determined from the intensity of the signal at δ -1.24 (12H) due to the TEMPO ligand relative to that at δ -1.59 (12H) due to the 4-oxo-TEMPO ligand. Acknowledgment. The authors thank Mr. J. Tsuchiya for his generous collaboration in EPR measurements. JA980273I
